Overlay proofs are extremely useful to the printing industry. They provide a means for predicting the quality of the printed image which is to follow. Overlay proofs are made by sequentially and separately forming single color proof images and laying them one over another in register. This provides a proof of the final printed image.
The use of individual color proof images in forming the composite has distinct advantages. The proof of each color can be viewed, and corrections of single colors can be readily made. The proofs can also be used in pairs to judge two colors, or in threes to compare 3 color proofs. On the other hand, the separation of color onto individual films produces an adverse effect on the total color rendition. The overlay sheets produce a shift in apparent color rendition in which the topmost colors are relatively dominant and the lower colors relatively weakened. This effect apparently is in part due to the multiple resin and air interfaces between the sheets which exhibit large differences in refractive indices.
Some printers have found it useful to temporarily laminate the four color proof films together by applying a non-adhesive liquid, such as water, between the films. This helps reduce the color rendition problems, but is messy and can produce spotty color variations if the liquid does not form a continuous coating or leaves by evaporation. The use of solvent type liquids can also produce other problems such as smearing the half-tone images. The volatile solvents would have more of a tendency for spotting and the more viscous, oil-like solvents would be very messy.